We had to replace our sewer line this year. All I can say is shop around...but also check references. We had a little more time than you so we were able to play hardball until someone gave us a price we felt was "reasonable"

If you can call $7,800 reasonable. But at first it was over 10. So I was happy with it in the end. We didn't have issues with a tree, but we did have a leaking pipe into our yard. Our home was built in 1910....the sewer line was beyond old and the cast iron pipe rotted leaving a nice big hole in the middle. We ended up having to replace the entire line from the house to the street. That meant concrete work, which was our largest part of the expense.
I am sorry you are going through all this. I feel for you. Hope things get better for you soon.